1. Field of the Invention
This invention relates to a holder for a tool such as a tap, an end mill, a drill, an internal cutting tool, or an abrasive grindstone.
2. Prior Art
In machining by a machine tool such as a milling machine, a boring machine, a lathe or a polishing machine, the working accuracy is influenced much by the presence or absence and the magnitude of imbalance in weight, in a circumferential direction, of a rotary shaft (that is, a spindle) of a machine tool, a rotating tool or the like. Therefore, in machining of the type mentioned, it is important to reduce the imbalance of a rotary shaft and a rotating tool as far as possible in order to raise the working accuracy.
As one of tool holders which can adjust a rotary shaft and a rotating tool against imbalance, a tool holder is available wherein a holder body having a first section in the form of a bar to be attached to a machine, a second section in the form of a bar coaxially extending from one end of the first section and a flange provided at a boundary portion between the first and second sections is used and a plurality of threaded holes which are opened toward the side of the second section are formed at intervals in a circumferential direction in the flange while screw members are screwed into one or more of the threaded holes in accordance with a direction of imbalance to be eliminated (U.S. Pat. No. 5,096,345, No. 5,033,923 and No. 5,125,777).
However, with the tool holder described above, since the flange thereof to be gripped by an automatic tool exchanging machine which automatically attaches and removes a tool to and from a machine tool together with the tool holder has threaded holes formed therein, the second section of the holder body makes an obstruction to an operation for forming the threaded holes in the flange, and as a result, it is difficult to form a plurality of threaded holes at angularly equal distances and imbalance in weight occurs with the holder body itself.
As another one of tool holders, a tool holder is available wherein a balance ring for balancing is attached to the flange of the holder body (WO95/26258). With this tool holder, since a recess for receiving the balance ring is opened to the first section side at which the tool holder is attached to a rotary shaft of a machine tool, the first section of the holder body makes an obstruction to an operation for forming such a recess on the flange, and as a result, it is difficult to form such a recess accurately and precisely and imbalance in weight occurs with the holder body itself.
Further, with the second-mentioned conventional tool holder described above, since the balance ring is attached to the attaching side to the rotary shaft, before the tool holder is attached to the rotary shaft, the balance ring must be attached to the tool holder and a screw for balancing must be attached to the balance ring from the rotary shaft side of the machine tool. Accordingly, it is difficult to balance the rotary shaft, and particularly after the tool holder is attached to the rotary shaft, it is very difficult to perform balancing.